Non-contact electromagnetic locating and tracking systems are well known in the art, with an exceptionally broad spectrum of applications, including such diverse topics as military target sighting, computer animation, and precise medical procedures. For example, electromagnetic locating technology is widely used in the medical field during surgical, diagnostic, therapeutic and prophylactic procedures that entail insertion and movement of objects such as surgical devices, probes, and catheters within the body of the patient. The need exists for providing real-time information for accurately determining the location and orientation of objects within the patient's body, preferably without using X-ray imaging.
U.S. Pat. Nos. 5,391,199 and 5,443,489 to Ben-Haim, which are assigned to the assignee of the present patent application and whose disclosures are incorporated herein by reference, describe systems wherein the coordinates of an intrabody probe are determined using one or more field sensors, such as Hall effect devices, coils, or other antennae carried on the probe. Such systems are used for generating three-dimensional location information regarding a medical probe or catheter. A sensor coil is placed in the catheter and generates signals in response to externally-applied magnetic fields. The magnetic fields are generated by a plurality of radiator coils, fixed to an external reference frame in known, mutually-spaced locations. The detected and used to compute the location of the sensor coil. Each radiator coil is preferably driven by driver circuitry to generate a field at a known frequency, distinct from that of other radiator coils, so that the signals generated by the sensor coil may be separated by frequency into components corresponding to the different radiator coils.
PCT Patent Publication WO 96/05768 to Ben-Haim et al., which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference, describes a system that generates six-dimensional position and orientation information regarding the tip of a catheter. This system uses a plurality of sensor coils adjacent to a locatable site in the catheter, for example near its distal end, and a plurality of radiator coils fixed in an external reference frame. These coils generate signals in response to magnetic fields generated by the radiator coils, which signals allow for the computation of six location and orientation coordinates, so that the position and orientation of the catheter are known without the need for imaging the catheter.
U.S. Pat. No. 6,239,724 to Doron et al., whose disclosure is incorporated herein by reference, describes a telemetry system for providing spatial positioning information from within a patient's body. The system includes an implantable telemetry unit having (a) a first transducer, for converting a power signal received from outside the body into electrical power for powering the telemetry unit; (b) a second transducer, for receiving a positioning field signal that is received from outside the body; and (c) a third transducer, for transmitting a locating signal to a site outside the body, in response to the positioning field signal.
U.S. Pat. No. 5,425,382 to Golden, et al., whose disclosure is incorporated herein by reference, describes apparatus and methods for locating a catheter in the body of a patient by sensing the static magnetic field strength gradient generated by a magnet fixed to the catheter.
U.S. Pat. No. 5,558,091 to Acker et al., which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference, describes a magnetic position and orientation determining system which uses uniform fields from Helmholtz coils positioned on opposite sides of a sensing volume and gradient fields generated by the same coils. By monitoring field components detected at a probe during application of these fields, the position and orientation of the probe is deduced. A representation of the probe is superposed on a separately-acquired image of the subject to show the position and orientation of the probe with respect to the subject.
Other locating devices using a position sensor attached to a catheter are described in U.S. Pat. Nos. 4,173,228 to Van Steenwyk et al., 5,099,845 to Besz et al., 5,325,873 to Hirschi et al., 5,913,820 to Bladen et al., 4,905,698 to Strohl, Jr. et al., and 5,425,367 to Shapiro et al., the disclosures of which are incorporated herein by reference.
Commercial electrophysiological and physical mapping systems based on detecting the position of a probe inside the body are presently available. Among them, CARTO™, developed and marketed by Biosense Webster, Inc. (Diamond Bar, Calif.), is a system for automatic association and mapping of local electrical activity with catheter location.
Electromagnetic locating and tracking systems are susceptible to inaccuracies when a metal or other magnetically-responsive article is introduced into the vicinity of the object being tracked. Such inaccuracies occur because the magnetic fields generated in this vicinity by the location system's radiator coils are distorted. For example, the radiator coils' magnetic fields may generate eddy currents in such an article, and the eddy currents then cause parasitic magnetic fields that react with the field that gave rise to them. In a surgical environment, for example, there is a substantial amount of conductive and permeable material including basic and ancillary equipment (operating tables, carts, movable lamps, etc.) as well as invasive surgery apparatus (scalpels, catheters, scissors, etc.). The eddy currents generated in these articles and the resultant electromagnetic field distortions can lead to errors in determining the position of the object being tracked.
It is known to address the problem of the interference of static metal objects by performing an initial calibration, in which the response of the system to a probe placed at a relatively large number of points of interest is measured. This may be acceptable for addressing stationary sources of electromagnetic interference, but it is not satisfactory for solving the interference problems induced by moving magnetically-responsive objects.
U.S. Pat. No. 6,373,240 to Govari, entitled, “Counteracting metal presence in a magnetic tracking system,” which is assigned to the assignee of the present patent application and is incorporated herein by reference, describes an object tracking system comprising one or more sensor coils adjacent to a locatable point on an object being tracked, and one or more radiator coils, which generate alternating magnetic fields in a vicinity of the object when driven by respective alternating electrical currents. For each radiator coil, a frequency of its alternating electrical current is scanned through a plurality of values so that, at any specific time, each of the radiator coils radiates at a frequency which is different from the frequencies at which the other radiator coils are radiating.
The sensor coils generate electrical signals responsive to the magnetic fields, which signals are received by signal processing circuitry and analyzed by a computer or other processor. When a metal or other field-responsive article is in the vicinity of the object, the signals typically include position signal components responsive to the magnetic fields generated by the radiator coils at their respective instantaneous driving frequencies, and parasitic signal components responsive to parasitic magnetic fields generated because of the article. The parasitic components are typically equal in frequency to the instantaneous frequency of the driving frequency, but are shifted in phase, so that the effect at each sensor coil is to produce a combined signal having a phase and an amplitude which are shifted relative to the signal when no field-responsive article is present. The phase-shift is a function of the driving frequency, and so will vary as each driving frequency is scanned. The computer processes the combined signal to find which frequency produces a minimum phase-shift, and thus a minimum effect of the parasitic components, and this frequency is used to calculate the position of the object. Varying the driving frequency until the phase shift is a minimum is described as an effective method for reducing the effect of field-responsive articles on the signal.
U.S. Pat. No. 6,172,499 to Ashe, whose disclosure is incorporated herein by reference, describes a device for measuring the location and orientation in the six degrees of freedom of a receiving antenna with respect to a transmitting antenna utilizing multiple-frequency AC magnetic signals. The transmitting component consists of two or more transmitting antennae of known location and orientation relative to one another. The transmitting antennae are driven simultaneously by AC excitation, with each antenna occupying one or more unique positions in the frequency spectrum. The receiving antennae measure the transmitted AC magnetic field plus distortions caused by conductive metals. As described, a computer then extracts the distortion component and removes it from the received signals, providing the correct position and orientation output.
U.S. Pat. No. 6,246,231 to Ashe, whose disclosure is incorporated herein by reference, describes a method of flux containment in which the magnetic fields from transmitting elements are confined and redirected from the areas where conducting objects are commonly found.
U.S. Pat. No. 5,767,669 to Hansen et al., whose disclosure is incorporated herein by reference, describes a method for subtracting eddy current distortions produced in a magnetic tracking system. The system utilizes pulsed magnetic fields from a plurality of generators, and the presence of eddy currents is detected by measuring rates of change of currents generated in sensor coils used for tracking. The eddy currents are compensated for by adjusting the duration of the magnetic pulses.
U.S. Pat. Nos. 4,945,305 and 4,849,692 to Blood, whose disclosures are incorporated herein by reference, describe tracking systems that circumvent the problems of eddy currents by using pulsed DC magnetic fields. Sensors which are able to detect DC fields are used in the systems, and eddy currents are detected and adjusted for by utilizing the decay characteristics and the amplitudes of the eddy currents.
U.S. Pat. No. 5,600,330 to Blood, whose disclosure is incorporated herein by reference, describes a non-dipole loop transmitter-based magnetic tracking system. This system is described as showing reduced sensitivity to small metallic objects in the operating volume.
European Patent Application EP 0-964,261 A2 to Dumoulin, whose disclosure is incorporated herein by reference, describes systems for compensating for eddy currents in a tracking system using alternating magnetic field generators. In a first system the eddy currents are compensated for by first calibrating the system when it is free from eddy currents, and then modifying the fields generated when the eddy currents are detected. In a second system the eddy currents are nullified by using one or more shielding coils placed near the generators.
U.S. Pat. No. 5,831,260 to Hansen, whose disclosure is incorporated herein by reference, describes a combined electromagnetic and optical hybrid locating system that is intended to reduce the disadvantages of each individual system operating alone.
U.S. Pat. No. 6,122,538 to Sliwa, Jr. et al., whose disclosure is incorporated herein by reference, describes hybrid position and orientation systems using different types of sensors including ultrasound, magnetic, tilt, gyroscopic, and accelerometer subsystems for tracking medical imaging devices.
Article surveillance systems using soft magnetic materials and low frequency detection systems have been known since the Picard patent (Ser. No. 763,861), which is incorporated herein by reference, was issued in France in 1934. Surveillance systems based on this approach generally use a marker consisting of ferromagnetic material having a high magnetic permeability. When the marker is interrogated by a magnetic field generated by the surveillance system, the marker generates harmonics of the interrogating frequency because of the non-linear hysteresis loop of the material of the marker. The surveillance system detects, filters, and analyzes these harmonics in order to determine the presence of the marker. Numerous patents describe systems based on this approach and improvements thereto, including, for example, U.S. Pat. Nos. 4,622,542 and 4,309,697 to Weaver, 5,008,649 to Klein, and 6,373,387 to Qiu et al., the disclosures of which are incorporated herein by reference.
U.S. Pat. No. 4,791,412 to Brooks, whose disclosure is incorporated herein by reference, describes an article surveillance system based upon generation and detection of phase shifted harmonic signals from encoded magnetic markers. The system is described as incorporating a signal processing technique for reducing the effects of large metal objects in the surveillance zone.
U.S. Pat. Nos. 6,150,810 to Roybal, 6,127,821 to Ramsden et al., 5,519,317 to Guichard et al., and 5,506,506 to Candy, the disclosures of which are incorporated herein by reference, describe apparatus for detecting the presence of ferrous objects by generating a magnetic field and detecting the response to the field from the object. A typical application of such an apparatus is detection and discrimination of objects buried in the ground. U.S. Pat. No. 4,868,504 to Johnson, whose disclosure is incorporated herein by reference, describes a metal detector for locating and distinguishing between different classes of metal objects. This apparatus performs its analysis by using harmonic frequency components of the response from the object.
U.S. Pat. No. 5,028,869 to Dobmann et al., whose disclosure is incorporated herein by reference, describes apparatus for the nondestructive measurement of magnetic properties of a test body by detecting a tangential magnetic field and deriving harmonic components thereof. By analyzing the harmonic components, the apparatus calculates the maximum pitch of the hysteresis curve of the test body.
An article by Feiste KL et al. entitled, “Characterization of Nodular Cast Iron Properties by Harmonic Analysis of Eddy Current Signals,” NDT.net, Vol. 3, No. 10 (1998), available as of May 2002 at http://www.ndt.net/article/ecndt98/nuclear/245/245.htm, which is incorporated herein by reference, describes applying harmonic analysis to nodular cast iron samples to evaluate the technique's performance in predicting metallurgical and mechanical properties of the samples.
There is a need for a straightforward, accurate, real-time method that addresses the problem of interference induced in electromagnetic locating and tracking systems caused by the introduction of non-stationary metallic or other magnetically-responsive articles into the measurement environment.